This application is based on and claims the benefit of European Patent Application No. 03 290 071.4 filed Jan. 10, 2003, which is incorporated by reference herein.
The present invention is generally concerned with mobile communication systems.
Mobile communication systems in a general way are subject to standardisation; therefore for a more detailed description of such systems one may refer in particular to the corresponding standards, published by the corresponding standardisation bodies.
Briefly, the general architecture of such systems is divided into a Radio Access Network (RAN), mainly responsible for transmission and management of radio resources on the radio interface between Mobile Stations (MS) and the network, and a Core Network (CN), mainly responsible for switching and management of the communications.
The evolutions of technology in such systems generally lead to a distinction between second generation systems and third generation systems.
A typical example of a second generation system is GSM (<<Global System for Mobile communication>>). The radio access technology used by the GSM RAN is based on multiple access techniques of FDMA/TDMA type (where FDMA stands for <<Frequency Division Multiple Access>> and TDMA stands for <<Time Division Multiple Access>>). The GSM RAN is composed of subsystems called BSS (<<Base Station Subsystems>>) and the GSM CN includes network elements such as MSC (<<Mobile Switching Center>>) and GMSC (<<Gateway Mobile Switching Center>>).
Initially, GSM was mainly intended for providing real-time services such as in particular telephony services, based on circuit-switched technology. GSM has next evolved towards providing non real-time services, such as in particular data transfer services, based on packet-switched technology. This evolution was made possible thanks to the introduction of GPRS (<<General Packet Radio Service>>), including the introduction of two new network elements in the CN, i.e. SGSN (<<Serving GPRS Support Node>>), and GGSN (<<Gateway GPRS Support Node>>). It is recalled that packet-switched technology enables a more efficient use of available resources, by sharing resources at any instant between different users.
A typical example of a third generation system is UMTS (<<Universal Mobile Telecommunication System>>). UMTS offers third generation services, including high bit-rates for both real-time and non real-time services. The radio access technology used by the UMTS RAN is based on multiple access techniques of CDMA type (where CDMA stands for <<Code Division Multiple Access>>). The UMTS RAN is also called UTRAN (<<UMTS Terrestrial Radio Access Network>>) and the UMTS CN includes network elements relating to packet-switched (PS) domain and network elements relating to circuit-switched (CS) domain.
Now, a further evolution of GSM is towards offering third generation services. A first step of this evolution corresponds to the introduction of EDGE (<<Enhanced Data rates for GSM evolution>>) enabling higher bit-rates on the radio interface thanks to the use of modulation techniques of higher spectral efficiency. A second step of this evolution corresponds to the support of packet-based real-time services.
When packet-based technologies are used, the Quality of Service (QoS) becomes an important issue. The QoS architecture in third generation systems is defined in the 3GPP TS 23.107 specification published by 3GPP (<<3rd Generation Partnership Project>>). This QoS architecture relies on different Bearer Services characterized by different QoS attributes including: traffic class, maximum bitrate, guaranteed bitrate, transfer delay, traffic handling priority, . . . etc. Further, a distinction is made between four classes of traffic, respectively:conversational, streaming, interactive, background. Conversational and streaming classes are mainly used for real-time traffic flows, for which the QoS requirements are the highest, in terms of guaranteed bit rate and transfer delay.
The present invention is more particularly concerned with the support of services, in particular real-time services, in the Packet Switched (PS) domain in such systems, in particular when considering GERAN access technology (where GERAN stands for <<GSM/EDGE Radio Access Network>>).
The general architecture of a system using GERAN access technology and packet-switched domain is recalled in FIG. 1.
The protocol architecture when considering GERAN access technology and Packet-Switched (PS) domain is recalled in FIG. 2.
The protocol layers at the radio interface, or interface between MS and BSS, or <<Um>> interface, include:                a first layer, or physical layer,        a second layer, or data link layer, in turn divided into different layers: according to increasing levels, MAC (<<Medium Access Control>>), RLC (<<Radio Link Control>>) and LLC (<<Logical Link Control>>, the BSS only being used as a relay function between MS and SGSN, for the LLC layer).        
In the same way, the protocol layers at the interface between BSS and SGSN, or <<Gb>> interface, include:                a first layer, or physical layer,        a second layer, or data link layer, in turn divided into different layers: according to increasing levels, <<Network service>>, BSSGP (<<BSS GPRS Protocol>>), and LLC (<<Logical Link Control>>, the BSS only being used as a relay function between MS and SGSN, for the LLC layer).        
Besides, higher level protocols (not illustrated specifically in this figure) are provided, at application level, or for management tasks such as MM (<<Mobility Management>>), SM (<<Session Management>>), . . . etc.
It is also recalled that frames called LLC frames are formed, in the LLC layer, from data units of higher level. In the LLC frames these data units are called LLC-PDU (<<LLC-Protocol Data Units>>) data units. LLC-PDU data units are then segmented in the RLC/MAC layer, so as to form blocks called RLC data blocks. RLC data blocks are then put in the required format for transmission on the <<Um>> interface, in the physical layer.
It is also recalled that before any data can be transferred for a MS in a packet data session, a PDP (Packet Data Protocol) context must be activated or created for this session, both in the MS and in the SGSN, this PDP context including routing information and QoS information for this session.
Once this PDP context is activated, the MS may transfer data. When a MS effectively has data to transfer during this session, it has to enter a mode (called Packet Transfer Mode) where there is a TBF (Temporary Block Flow) established for this MS, i.e where this MS is allocated radio resource on one or more PDCH (Packet Data Channel) for the transfer of LLC PDUs. Otherwise, when the MS has no data to transfer, it is in a mode (called Packet Idle Mode) where it is not allocated any resource on a PDCH.
The process by which a MS is allocated radio resource on one or more PDCHs is called TBF establishment. Briefly recalled, this process may be either in a one-phase access or in a two-phase access. In either case the MS sends a Packet Channel Request to the network. In one-phase access, the network responds by reserving radio resources for data transfer for this MS. In two-phase access, the network first responds by reserving radio resources for the MS to transmit a more detailed description of its needs, and thereafter reserves radio resources for the data transfer for this MS.
As recalled above, higher data rates can now be achieved thanks to the GPRS enhancement corresponding to EDGE, also called EGPRS (Enhanced GPRS). Another way of achieving higher data rates is via multislot operation, whereby a MS can be simultaneously allocated more than one PDCH. However such ways of achieving higher data rates are generally not supported by all MSs and/or all cells of the network. Therefore, in order for the network to act efficiently, some mechanisms are required, by which the network can get a knowledge of the radio access capabilities of a MS, including in particular its capability of operating in EGPRS mode, and/or its multislot class (or number of timeslots on which the MS can operate simultaneously).
It is also recalled that before requiring any activation of a PDP context, a MS has to perform a GPRS Attach procedure, by which it provides the network with its identity as well as other parameters, mainly for a purpose of checking whether the user is authorised to have access to GPRS services, depending on his subscription. Among those parameters, the MS provides its radio access capabilities.
A typical transaction where the network has to get a knowledge of the MS radio access capabilities is the TBF establishment procedure. As this transaction is between the MS and the BSS, specific mechanisms have been provided to enable the BSS to get a knowledge of the MS radio acces capabilities, such mechanisms in particular providing that a different request message is sent by the MS depending on whether it supports EGPRS or not (EGPRS Packet Channel Request message if the MS supports EGPRS, or (Packet) Channel Request message if the MS does not support EGPRS, the latter message depending on whether PBCCH (Packet Broadcast Control Channel) is present in the cell or not), or that the MS multislot class is indicated in the request message sent by the MS.
Turning back to the QoS architecture required for supporting third generation services in a system such as the one recalled at FIG. 1 (including the support of high bitrates for real-time services in the packet-switched domain), it is recalled that the setting-up of a bearer in such a system is generally performed in a way as to guarantee that the QoS requirements are fulfilled at different levels of the system, taking into account the different characteristics of each level. The different bearers on which the QoS architecture relies include in particular a radio bearer, and the QoS requirements have to be fulfilled at the radio level.
Therefore, when considering GERAN access technology, the support of services such as in particular real-time services, in the Packet Switched domain requires several basic functions:                support of Rel-99 GERAN standards in the MS, BSS, and SGSN,        support of Rel-99 QoS parameter negotiation at PDP context activation time, including a negotiation with the BSS (in Rel-97, the QoS parameters are negotiated only between the MS and the SGSN). This negotiation between the BSS and the SGSN requires the support of the Packet Flow Context feature on the Gb interface (defined in 3GPP TS 08.18),        support of specific Call Admission Control algorithms in the BSS and the SGSN in order to guarantee real-time constraints such as transfer delay and bitrate, which requires the reservation of resources at the time of a bearer set-up.        
FIG. 4 shows an overview of the various steps involved in setting-up a bearer such as for example a real-time bearer.
1) The R99 MS requests the activation of a PDP context, for which the “QoS Requested” parameters correspond to a real-time bearer.
2) The SGSN may then perform security and trace functions. A Call Admission Control algorithm is called to check whether the required QoS attributes can be fulfilled. The SGSN may then restrict the requested QoS attributes given its capabilities and the current load, and it shall restrict the requested QoS attributes according to the subscribed QoS profile. The SGSN then requests the creation of the PDP context in the GGSN.
3) Various functions are performed in the GGSN, which may even reject the request from the SGSN if the QoS Negotiated received from the SGSN is incompatible with the PDP context being activated.
4) Once the creation of the PDP context in the GGSN has been confirmed as successful, the SGSN then requests the creation of a Packet Flow Context (PFC) for the real-time bearer. Although it is possible in theory to aggregate several bearers into the same PFC, it seems better to create one PFC for each real-time bearer and aggregate only non real-time bearers having similar QoS characteristics within the same PFC. The request from the SGSN contains several mandatory parameters:                TLLI: identifier of the Mobile Station        PFI: Packet Flow Identifier (identifier of the PFC)        PFT: Packet Flow Timer (lifetime of the PFC during periods of inactivity)        ABQP: Aggregate BSS QoS Profile (QoS parameters characterising the PFC)        
5) The BSS then performs a Call Admission Control algorithm to check whether the requested QoS attributes can be fulfilled. Several functions may be performed in order to be able to support the requested QoS (e.g. reallocation of other MSs, redirection of the MS to another less loaded cell, etc). The BSS may restrict the requested aggregate BSS QoS profile given its capabilities and the current load, although not fulfilling the guaranteed bitrate and the transfer delay attributes should as far as possible be avoided. The BSS performs resource reservation in order to support the negotiated guaranteed bitrate and transfer delay, taking into account the RLC mode that will be used for the flow (quite likely: RLC acknowledged mode since LLC PDUs should be rather large: 500 octets or more for video streaming for instance). The resources reserved on the radio interface need to be higher than the negotiated guaranteed bitrate due to radio interface overheads.
6) The BSS acknowledges the PFC creation if it is successful, providing to the SGSN the negotiated ABQP, i.e. the negotiated Quality of Service attributes.
7) Assuming that the negotiated ABQP is acceptable for the SGSN, the PDP context activation procedure is completed by the sending of an acceptance message to the Mobile Station.
8) Because the SGSN will have to comply with the announced leak rate for the corresponding MS/PFC, it is quite likely that the BSS has to send a FLOW CONTROL MS or FLOW CONTROL PFC message to the SGSN in order to announce a leakrate that is greater than the negotiated guaranteed bitrate (otherwise default values are used). The choice between MS and PFC flow control is implementation dependent and depends also on whether there are other active PFCs for the same MS.
9) The SGSN acknowledges the FLOW CONTROL MS or FLOW CONTROL PFC message.
10) When the real-time session is started (case of downlink flow in this example) thanks to other upper layer protocols not described in this document, the SGSN sends to the BSS, BSSGP PDUs containing the PDU lifetime, the QoS profile (R97, not useful in this case), the MS Radio Access Capabilities, the PFI and the LLC PDU to be sent.
11) The BSS sends the LLC PDUs to the MS.